The reviews I saw on your Pentax A 28-80 sounds like it's a so-so lens, not the greatest but not the worst, either. See http://stans-photography.info/
for reviews on various Pentax lenses. I'm not familiar with your Sigma lens. What did you think of it when you were shooting film? If youdidn't likeit when you were shooting film, you probably won't like it all that much when shooting digital.
You are correct about the sensor being smaller and not using the far edges of a full-frame lens. When I was shooting film I never used my K-mount 24mm lens, but found that it was much more useful on a dSLR.
I happily shoot Pentax (I have the K100 and K20, having sold a K10 when I upgraded). They are rugged cameras, with the K200 and K20 being weather sealed - useful for snow and desert use (I shoot in both). I've read that the Nikon D40 and Pentax K100 both have the same 6 mp sensor, which has been a real work-horse and one that I really like (notice I kept my K100 when I upgraded to the K20). However, the D40 has fewer focus points so would be at a disadvantage shooting sports.
The Pentax K200 has the same 10 mp sensor that Nikon used in the D80 and D60, so images would be similar, though each manufacturer processes the image in different ways. Make sure you check out the sample pictures and see if you prefer one look over the other.
I'm not interested in video (any video I've taken has been very boring, I just don't think that way), so the D90 doesn't interest me at all.
All of these cameras are different sizes and have different ergonomics. You really need to handle all of the ones you are considering to see if you would be happy carrying them around all day. Do they fit your hands - are they too small, too large, too heavy, too light? The best camera in the world can't take good pictures if it's stuffed in your closet because you hate carrying it (a friend of mine bought a D40 because he found carrying the D200 all the time a big bother).